Optical transmission systems have evolved to multiplexed systems that transmit a plurality of optical main carrier channels on a single optical fiber transmission line, as in wavelength division multiplexing (WDM) or dense WDM (DWDM). While each WDM channel is capable of transmitting large volumes of data at very high speeds, channel bandwidths are often underutilized by relatively slow data streams.
Subcarrier modulation has been successfully used in radio frequency (RF) transmissions to combine both digital and analog information onto a main carrier frequency transmission. Typically RF subcarriers are created at fixed frequency intervals from the center frequency of the main RF channel. Lower bandwidth information can then be coded or modulated onto the subcarriers and thus fill the sub-band channels. A main carrier transmission can include a plurality of subcarriers, with their associated sub-bands.
WDM channel bandwidth capacity could be used far more efficiently by the addition of optical sub-bands. But, while there are similarities in the underlying fundamental modulation theory between electrical RF subcarrier systems and optical subcarrier systems, the technology for implementing optical subcarriers and sub-bands is quite different and heretofore has been unavailable. An optical apparatus for adding optical subcarriers to main WDM optical carriers is needed.